You rebuild the calipers with new seals, clean pistons, new braided lines, rebuilt master cylinder and new pads & fluid, bleed, bleed, bleed, and yet still the lever pulls back to the bar????
WHY??
There are many theorys as to why?
The main reason for spongy brakes is air in the system, BUT you have bled them again and again! Bled them on the bike & bled them off the bike, you can bleed them from the nipple and the banjo joints but still spongy brakes.
Other theories are contaminated or wrong pads, faulty brakelines (especially if they are oem rubber ones), damaged seals, worn master cylinder, faulty calipers? the list could go on.Here's a few comments i've picked up around the net which you may want to consider.
those damn calipers use to give racers fits back in the day, he says basically its a design flaw. The pad is too big, and once you get them hot enough the backing plates warp, and once they are warped, you get a musshy lever, becuae you waste all of your lever travel strightening the pads out. This all seems conssitent with my experience, as the lever felt its best when I put the new pads in, prior to warping them. He said it wasn't uncommon for racers to throw a new set of pads in for every race weekend.
His only suggestions were to run an organic pad. Ferodo 911 was a good option. Organic runs cooler then sintered. Makes sense. He also said I could use a brembo radial master. The bigger bore would allow me more travel so I can stighten the bent pads out and still have travel to develop pressure to stop!
take the pads out and push the pistons all the way in. with the calpiers mounted and your discs tight against one side or the other of the disc carriers, measure the clearance at the leading and trailing edge of the caliper. if these figures are not the same, your caliper is not sitting 'square' in their mounts. shim them appropriately. the larger the caliper, the more it can be an issue.
I got sick and tired of it like you and invested in a 19x20 Brembo MC......WOW what a difference!!! The braking system is money now! Dead on balls consistant at EVERY stop under ANY condition. Everything stayed the same minus the MC, its like riding a different bike!
I wholeheartidly believe my stock MC could not supply the proper amount of fluid at the proper pressure level to keep up with the 6pots. Being the Brembo can probably stop a semi, there is no problem now.
I had a similar problem on me ZZR afore I sold it. I eventually traced it to the rubber diaphragm under the lid on the master cylinder. After I'd pumped the brakes up, it was acting like an elastic band and sucking all the fluid back up into the reservoir. Try taking the cap off, pumping up your brakes THEN putting the cap back on and see if that works.
One guy I spoke to said the Tokico body flexes under pressure although I'm not sure how much truth there is in that.